Jet-electrolytic method of configuring bodies



A. R. TOPFER JET-ELECTROLYTIC METHOD OF' CONFIGURING BODIES Nov. 17, 1959 Filed May 2, 1957 WIJ/wz United States @arent M JET-ELECTROLYTIC METHGD 0F CONFIGURING BODIES Alvin R. Topfer, Ambler, Pa., assigner to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania Application May 2, l1957, Serial No. 656,728

7 Claims. (Cl. 21M- 143) The present invention relates to methods for the removal of material from electrically-conducting bodies, and particularly to electrolytic methods for the shaping of such bodies.

Methods are known in the prior art for effecting accurate shaping of bodies of material by applying a jet of electrolyte to the surface of a body while maintaining the body positive with respect to the jet of electrolyte. For example, in the copending application Serial No. 472,824 of I. W. Tiley and R. A. Williams, led December 3, 1954, and entitled semiconductive Devices and Methods for the Fabrication Thereof, there is de` y ananas Patented Nov. l?, 1959 be monitored during the jet-etching process itself by passing certain wavelengths of infra-red radiation through the portion of the body being jet-etched, and detecting certain clnfrges in the components transmitted through the semiconductive material beneath the jet. llt has been found that if the pit produced by the jet-etching has a highly-curved bottom surface, the incident infra-red radiations tend to be diffused optically in the body and the amount of infra-red energy emanating normally to the opposite surface of the body is reduced below that which Would result if the bottom of the pit Were more nearly flat. Accordingly, the detection of the infra-red energy and the control of jet-etching thereby are 'substantially more difficult when the bottom of the etch pit is highly curved.

It is also often desirable, as in the processing of semiconductive bodies for transistor purposes, that the surscribed in detail a jet-electrolytic etching process for i accurately configuring a semiconductive body, in which the body is maintained positive with respect to the jet of electrolyte. In -such processes it is generally desirable that the removal of material by the etching occur as rapidly as possible, since the time consumed in removing the material may be responsible for a substantial portion ofthe total cost of any device produced by the process, particularly when the process is utilized in mass production. It is also often desirable that the bottom of the pit produced by such removal of material be substantially dat-bottomed, for several reasons. Considering first applications of the method to the fabrication of transistor devices having emitter and collector elements substantially parallel to the opposed surfaces of the thinned region of a lsemiconductive body, as in the surface-barrier transistor described in the copending application Serial No. 472,826 of R. A. Williams and l. W. Tiley, filed December 3, 1954, and entitled Electrical Device, the high frequency performance of such devices is usually enhanced by providing the emitter and/ or collector barriers adjacent substantially llat, plane-parallel surfaces of the semiconductive material. While the emitter and collector barriers may be made substantially plane-parallel, even though the pits have substantial curvature compared to their diameters, by making the area of the bottoms of the: pits large and utilizing only a small part of the bottom area of each pit for the active element of the transistor, this expedient requires removal of an unnecessarily large amount of material, thereby adding to the expense of the manufacturing process. However, if the pit can be made initially with a relatively flat bottom and relatively steep sides, the desired planeparallel configuration of material can be obtained by the removal of less material and with consequent commercial advantage. In addition, a flat-bottomed, steep-sided geometry of pit produces a lower base resistance in transistors made therefrom.

Flat-bottomed pits in semiconductive materials have also been found desirable from another viewpoint when the depth of the depressions is determined by infra-red thickness control such as that described in the copending application Serial No. 449,347 of Robert N. Noyce, led August l2, 1954, and entitled Electrical Method and Apparatus. As described in that application, the thickness of the material remaining under the pit may face of the semiconductive material subjected to the etching process remain smooth, and, in fact, it is often desirable that the etching action be such as to reduce any surface roughnesses originally present, thus eliminating or reducing the need for chemical etching prior to electrolytic jet-etching.

The foregoing characteristics of a material removal process are especially desirable in the electrolytic machining of germanium since this material is most widely used in transistors and is especially well adapted to control of its thickness by infra-red methods. However, rapid, localized removal of material electrolytically is of general utility as a machining method regardless of the nature of the material and, for example, is useful in configuring metal bodies as well as semiconductive bodies.

Accordingly, it is an object of my invention to provide an improved method for the electrolytic removal of material from a body under treatment.

Another object is to provide an improved method for the jet-electrolytic configuring of bodies of material such as germanium, arsenic or antimony.

Another object is to provide an improved method for the jet-electrolytic removal of material from a body of germanium, which operates at a higher rate than prior art methods. It is another object to provide a method forV the rapid yet smooth electrolytic machining of materials such as germanium, arsenic or antimony.

Still another object is to provide a novel electrolytic method for producing pits having substantially flat bottoms in germanium, and particularly in lll-oriented germanium. Y

It is a further object to provide a method for the jetelectrolytic removal of material from a germanium body under treatment which also operates to reduce any surface roughness which may exist prior to initiation of th treatment.

A still further object is to provide such a method of electrolytic material removal in ywhich the electrical supply requirements are especially compatible with those required for electroplating of the same body.

In accordance with the invention the above objectives are realized by the provision of a process in which a jet of electrolyte is directed against a body of a material such as germanium, arsenic or antimony, while a potential negative with respect to the jet is applied to the body so that the electrical current in the jet flows in a direction opposite to that normally employed in etching. The density of the current employed is in general substantially greater than is employed in conventional jet etching or plating, and is for example of the order of hundreds or thousands of amperes per square inch. Preferably, but not necessarily, the electrolyte is one having cations which are principally of a non-platable type, such as hydrogen. Under these conditions very rapid removal of material has been produced in the region of jet impingement. In the case of germanium such as is utilized in semiconductor devices, the rate of removal may readily be about six times greater than has been found practical with conventional jet-etching in which the semiconductive body is maintained positive with respect to the jet.

While the exact type of etching obtained in any particular application depends in some degree upon the nature of the material etched, the electrolyte used and the density of the electrical current ilowing through the jet, it has been found that by suitable adjustment of the operating conditions, depressions having very flat, smooth bottoms may readily be obtained, as is desirable for certain purposes such as those mentioned hereinbefore-namely, production of high frequency transistor devices and/or jet-etching of germanium with simultaneous infra-red measurement of thickness. Furthermore, it has been found that the process in some of its forms can be employed to smooth out roughnesses which may exist in the surface of the semiconductive body prior to initiation of the etching process, particularly in the case of lll-oriented germanium single crystals. This is especially advantageous in the manufacture of semiconductor devices in that semiconductor blanks which have been produced by sawing or similar abrasive method may be subiected to the present process to provide a localized region of precise small thickness therein without requiring a preliminary chemical etching to smooth the rough surfaces produced by the sawing action. In addition, the process can readily be performed without producing random etching or blast holes in the semiconductor material, as sometimes occurs in conventional etching processes.

The process of the invention also makes possible simplification of semiconductor shaping procedures, particularly in that strong illumination of the region subjected to treatment does not appear to be necessary for rapid, smooth removal of material and, where the material removal process is to be followed by an electroplating process, a single source of potential of predetermined polarity is sucient for both processes. For example, sequential local removal of material and local electroplating can readily be accomplished with the body maintained negative with respect to the electrolyte at all times by utilizing rst a large current tot produce etching and then a lower current in the same direction to produce plating.

As mentioned hereinbefore, to achieve the foregoing objectives the electrical current density is raised above a predetermined minimum value characteristic of the material and the electrolyte employed, typicaldensities being of the order of between one-hundred and onethousand amperes per square inch. For any particular material and electrolyte composition the optimum current density may readily be determined experimentally merely by increasing the current until the desired results are obtained. Although the method may be practiced by the application of a current of constant value between electrolyte and body, I have found it also advantageous in some circumstances to utilize a varying current, such as a rectified sine wave or other pulsatile waveform, having values in the current range required for the present type of material removed.

For convenience the term etching will be utilized herein to include removal of material by the method of the invention, although the mechanism of the removal appears to be greatly different from that involved in conventional etching. While all of the details of this mechanism are not fully understood, it-is believed that there is involved the formation of a hydride of the material being etched. Since germanium, arsenic and antimony can combine with nascent hydrogen during come more readily apparent from a consideration of the following `detailed description taken in connection with the accompanying drawings, in which:

electrolysis to produce hydrides, the process is highly Figure l is a diagram illustrating apparatus useful for practicing the invention in one of its forms;

Figures 2 and 3 are sectional views of bodies of germanium to which reference will be made in describing typical results of practicing my method; and

Figures 4 and 5 are diagrammatic representations illustrating arrangements for practicing the invention in two of its other forms.

Turning now to Figure l, the apparatus shown therein comprises one preferred arrangement which may be used in accordance with the invention rapidly to produce a pit in a wafer- 10 of single-crystalline germanium while monitoring the thickness of the material remaining beneath the pit by an infra-red measuring technique. It will be understood that the drawingis illustrative only, and that the various parts thereof are not necessarily to scale.

Wafer 10 in which the pit is to be produced is disposed between the jet-forming apparatus 12 and the gas-blast generator 14 so as to be impinged on its lower surface by the electrolytic jet 16. The function of generator 14 is merely to direct a drying blast of a gas such as nitrogen or air against the upper surface of Wafer 10, thereby to prevent wetting of this surface by the electrolyte which might interfere with stable infra-red transmission. To provide the desired gas blast, gas inlet 18 may be connected to a source (not shown) ot compressed nitrogen, which gas then iiows through orifice 20 onto the upper side of wafer 10.

.let-forming apparatus 12 is constructed so as to permit infra-red radiations from source 24 to be directed through the jet 16 to the region of body 10 impinged by the jet, while the gas-blast generator is constructed to accommodate and support an infra-red sensitive cell'26 disposed to detect infra-red radiations transmitted through body 10. Suction tubes 28 and 30, shown diagrammatically, may also be provided to produce regions of reduced air pressure adjacent the region of iet impingement, thereby to improve the stability and smoothness of the jet as described in the copending application Serial No. 637,972 of R. T. Vaughan, filed February 4, 1957, and entitled Method and Means for Fabricating Semiconductive Devices and the Like.

Germanium wafer 10 is in this case ohrnically soldered to a base tab 32, by which it is held between jet-forming means 12 and gas-blast generator 14. Jet-forming means 12, gas-blast generator 14 and wafer 10 may be supported in their proper relative positions by any appropriate means; although for convenience a common Lucite case 34 has been indicated in the drawing, it will be understood that, for greater accuracy in positioning of the several elements, precision-machined parts may be'used.

Electrolyte from electrolyte source and pump 36 supplies electrolyte under pressure to jet-forming apparatus 12 by way of inert tubing 38. In this example the iet forming apparatus comprises a chamber 40, closed at one end by the nozzle 42 which may be of ceramic and which contains a small orifice 44 from which the jet of electrolyte emanates. The general principles of electrolyte-circulating and jet-forming apparatus having been described in the above-cited copending application of Tiley and Williams, they need not be set forth in further detail herein.

To permit infra-red radiationsV to pass from infra-red source 24 through the jet-forming apparatus 12 and, by way of the jet and the semiconductive wafer 10, to the infra-red detector cell 26 located on the opposite side of the wafer, the electrolyte and any associated parts in the path between the source 24 and the wafer 10 are composed of materials which exlhibit substantial transmittance for infra-red radiations of the wavelength to be utilized in the measuring process.. It xyill be understood that 5 appropriate condensing lenses and collimating means (not shown) may be included in infra-red source 24 to direct radiations principally into the electrolyte in nozzle 42 and thence to the jet 16. r'

To facilitate measuring the infra-red radiations by the monitoring system there may be employed a light chopper 44 in the form of a peripherally-serrated disc 46 having its serrated portion interposed in the path of the infra-red radiations and rotated by a synchronous motor 48 as shown, whereby the radiations are periodically interrupted. Leads from infra-red detector 26 supply the output signal thereof to an amplifier 50 and indicator S2 which together are operative to produce an output indication which varies in predetermined fashion as the thickness of the semiconductive body approaches and attains the desired value.

In accordance with the invention the electrolyte impingent upon the under-surface of wafer 10 is maintained at a potential positive with respect to the wafer by means of potential source 56, one terminal of which is connected by way of switch 58 to the base tab 32, and the other terminal of which is connected by way of a variable resistor 60 to an inert metallic electrode 62 immersed in the electrolyte. Preferably the electrode 62 is located in Ia side-arm of the tubing such as 64 provided with a gas pressure release arrangement represented schematically as element 66, so that any bubbles formed about electrode 62 are prevented from passing into jet 16 and interfering with the infra-red thickness measurements. Potential source 56 may in some instances comprise a source of direct voltage, and in other cases a varying voltage such as a rectified half sine-wave. Variable resistor 60 permits adjustment of the current supplied to the jet by the potential source. It will be understood that, although the particular apparatus shown is especially useful in the specific application of the invention now being described, the only elements necessary to practice the invention in its broader forms are appropriate means for impinging a jet of electrolyte against the body and a potential source capable of supplying to the jet a sufficiently high current of the polarity employed in accordance with the invention.

To practice the method of the invention with the apparatus of Figure 1, the pump and suction devices are actuated so that the desired jet of electrolyte is impinged against the germanium wafer 10.' Switch 58 is closed and resistor 60 is adjusted to apply the appropriate positive potential difference between electrode 62 and wafer 10, and to cause the desired electrolytic current to flow through the jet. In the application of the invention shown, the infra-red source 243 and the associated light-chopper 44 simultaneously provide a beam of periodically-interrupted infra-red radiations which passes through the jet 16 and wafer 10 to the infra-red detection cell 26, and the resultant cell output signal is supplied to amplier S and indicating device 52.

The effect of the jet and the applied current is then swiftly and smoothly to remove material adjacent the region of impingement of the germanium, provided the electrolytic current is sufficiently high. By way of example, when the jet 16 is 16 mils in diameter, about 2 millimeters long and composed of 1.0 normal hydrochloric acid, when wafer 10 is of 2 ohm-centimeter, N- type germanium of 111 crystal orientation rand when the electrolytic current is about 170 milliamperes, the thickness of the germanium body beneath the jet decreases from about mils to about 0.2 mil in about 12 seconds, corresponding to a thickness reduction rate of about 0.4 mil per second. The average current density in the jet in this case is about 850 amperes per square inch. The resultant etch pit is smooth and essentially flat-bottomed, having the general form illustrated in Figure 2. As the desired ultimate thickness is approached, the infra-red radiations are transmitted etiiciently through the flat- 6 bottomed pit tothe infra-red detecting' apparatus to provide the desired accurate indications of the .remaining thickness of the germanium wafer.

The precise nature of the etching action obtained when this procedure is followed has been found to depend upon the nature of the body subjected to treatment, the magniture of the electrolytic current density employed and, to some extent, upon the nature of the electrolyte utilized. With regard to the nature of the material treated, the method has been found to be particularly advantageous when employed on germanium, although rapid removal of material has also been obtained with arsenic and antimony bodies. In the case of germanium, either N- or P-type material may beutilized and anyof the usual crystal orientations appear to be satisfactory.

Any of a variety of common electrolytes may also be utilized, including various normalities of sulphuric acid, hydrochloric acid and a mixture of hydrouoric acid and sodium uoride. Although the acids appear generally to be most convenient in readily producing rapid, smooth etching, non-acidic electrolytes such as potassium hydroxide and ammonium chloride have also been used to produce material removal. In some instances transient stimuli, such as a ash of bright light or a momentary change in the magnitude or direction of the current, have been found helpful in initiating the etching action should it be difficult to start with certain electrolytes or material.

As mentioned hereinbefore, the nature of the etching action obtained generally depends in substantial measure upon the electrolytic current density. In a typical case there is little or no removal of material at low current densities, e.g. of theorder of less than about amperes/ square inch, but as the current is increased beyond a threshold value characteristic of the particular application, rapid etching begins. VIn some cases the current density must be increased well beyond the threshold value before a regular or smooth-bottomed pit is etched. As the current density is increased the rate of etching generally continues to increase, but the temperature of the jet eventually increases greatly also, so much so that the germanium may be melted inthe region of jet impingement.

The precise relation between etching rate and electrical current density has been found to depend in some degree upon the nature of the electrolyte and the mate-rial treated, but the general form of the relationship has been found to be similar for many different applications. Thus in one series of experiments the material etched was N-type germanium, the jets were 15 mils in diameter and were of sulphuric acid of 0.4, 0.6, 1.0` and 1.5 normality in different cases. The etching current was half-Wave rectified, 60-cycle alternating current, measured with a direct-current meter indicating the average value of the current. The variations of etching rate with current were similar for al1 four normalities of electrolyte, the values given in the following table being typical.

TABLE I Average current density, Etching rate,

From the table it will be apparent that, for average currents of less than about 50 amperes/square inch, little etching occurred. Rapid etching occurred at 200 amperes/square inch, and increased with further increases in current until melting of the germanium was encountered at about 600 amperes/square inch in this instance.

In considering the above speciccurrent values it is significant that, since the current waveform was that-of a half-sinewave, the peak currents were about three times greater than the values listed. By using such a pulsatile waveform, the higher currents required for very rapid etching can be utilized with less tendency toward melting of the germanium.

The shape of the etched depression also depends upon the current employed. For example, in the foregoing experiments it was found that mounds or plateaus in the bottom of the depression often occur when the current is relatively low, which do not appear when higher currents are used.

The following examples of other specific applications of the invention are presented in the interest of complete deniteness without thereby intending to limit the scope of the invention.

Example I An electrolytic jet 16 mils in diameter of 1.0 Normal hydrochloric acid was directed from a distance of about 2 millimeters against a body of N-type germanium 5 mils thick, of about 3 ohm-centimeter resistivity and cut in the 111 crystal orientation. The voltage applied to the electrode 62 was 500 volts D.-C. positive with respect to the germanium body, and the distance from the anode to the body through the electrolyte was about 3 inches; the current through the jet was about 140 milliamperes D.C. The resultant etch rate was about 0.4 mil per second, so that the 5 mil body was etched to about 0.2 mil in 12 seconds, and the etch pit had the geometry shown in Figure 3 in which the bottom of the pit comprises a smoothsurfaced peripheral groove surrounding a central plateau having a relatively rough surface.

Example Il Increasing the current from 140 to 170 milliamperes but otherwise using the same conditions as in Example I, the smooth, flat-bottomed pit shown in Fig. 2 was obtained with an etching rate of about 0.4 mil per second.

Example III Utilizing 11G-oriented germanium, 0.75 Normal HCl and a current of about 125 milliamperes, but with conditions otherwise as in Examples I and II, rapid, localized etching occurred.

Example IV Utilizing 0.5 Normal HCl and a current of about 80 milliamperes but with conditions otherwise as in Example III, rapid and smooth etching was obtained.

Example V Utilizing the same arrangement of apparatus as in Example 1V, but employing P-type germanium instead of N-type germanium, and l Normal HCl, rapid etching at the rate of about 0.4 mil per second was obtained with a direct-current of about 120 milliamperes.

Example Vl Utilizing the same apparatus as in Example V, but

` with N-type material and l Normal NHlCl as the electrolyte, etching was obtained with half-sine wave currents of about 50-90 milliamperes R.M.Si.

Example VII With the same semiconductive material and waveform of current as in Example VI, but employing an electrolyte consisting of a mixture of 0.2 Normal NaF and 0.2 Normal HF, rapid but rough etching was obtained.

Example VIII Utilizing apparatus of the type shown in Figure l, with a jet of 0.4 Normal H2504 of 9 mils diameter, antimony was etched rapidly with a direct-current of 42 milliamperes.

Example IX Under the same conditions as were used in Example VIII, arsenic was similarly etched with a direct-current of 62 milliamperes.

The method has also been employed using non-aqueons electrolytes to ellect etching, although the etching usually proceeds at a lower rate. For example HCl in ethylene glycol was found to produce slow etching at about 500 amperes/square inch, D.C.

It willbe understood that combinations of jets may also be utilized Where desired, and operated in accordance with the invention. Thus in Figure 4 there is represented schematically an arrangement of a pair of electrolytic jets 100 and 102 directed against opposite surfaces of semiconductive wafer 104. In this case the potential source 106 supplies a positive potential to jet 102 by way of variable resistor 108 and switch 110, and at the same time supplies a positive potential to jet 100 by way of variable resistor 114 and switch 116. With appropriately high currents, rapid etching of a pair of opposed depressions in Wafer 104 then occurs. The following are exemplary operating conditions.

Example X Using a pair of opposed jets each 16 mils in diameter and composed of 0.4 Normal H2804, a wafer of N-type germanium was rapidly etched using a half-Wave rectied current of milliamperes total for the two jets together as measured by a D.C. meter, with the applied potential in the direction to make the jets positive with respect to the wafer.

In the modied arrangement shown in Figure 5, two opposed jets and 102 are used but only jet 102 is supplied with potential from source 106. With this arrangement it was found that, for moderately high currents, etching occurred under the unbiased jet 100 only; for a higher current, etching occurred under both jets; and for an even high current, etching occurred only under the biased jet 102. One example of such opera tion was as follows.

Example XI Using one jet of 8 mils diameter and 2 Normal H2SO4 and another, opposed jet of 16 mils diameter and 1 Normal H2504, and applying current to the 16 mil jet only, when the current was 100 milliamperes, etching was obtained under the 8 mil unbiased jet only; when the current was increased to milliamperes, etching was obtained under both jets; and with a current of to 270 milliamperes, etching was obtained under the l6-mil, biased jet only.

As to the mechanism of material removal by the process of the invention, it appears that the process involves the production of a hydride of the material under treatment; thus the process has been found effective to produce rapid removal of material from bodies of germanium, antimony and arsenic which form hydrides cathodically, but inelective on silicon, carbon, aluminum, zinc and other material which do not readily form hydrides cathodically. In addition, it has been found analytically that large quantities of germanium hydride are formed in the treatment of germanium by the process of the invention. However, in View of the extremely high current densities which are typically employed, the high temperatures produced in the area of reaction and the unusual electrolyte-dow conditions produced by the jet, it is not apparent that the chemical reaction taking place is a simple one and it 'appears more likely that the etching action involves an unusual combination of physical and chemical elects.

In any event, removal of germanium in the region of jet impingement by the method of the invention, involving the use of high currents applied in the direction opposite to that normally used in etching, has been found particularly advantageous yin reducing the time required 9 for, and facilitating the infra-red monitoring of, Ithe production of germanium bodies of reduced thickness suitable for the base regions of surface-barrier or alloy-junction transistors.

Although the invention has been described with particular reference to specific embodiments thereof, it will be understood that it may be practiced in a variety of forms differing from those exemplified without departing from the scope thereof.

I claim:

1. The method of removing a portion of a body of material selected from the class consisting of germanium, arsenic and antimony, comprising the steps of directing against said portion of said body a jet of an electrolyte containing hydrogen ions While maintaining said jet positive with respect to said body lto pass a current between said jet and said body, said jet being maintained sufficiently positive with respect to said body to produce in the portion of said jet impinging said body portion an average electrical current density of at least 50 amperes per square inch and to cause etching of said body portion by said jet.

2. A method in accordance with claim 1, in which the cations of said electrolyte are of a type and concensity is in a range having a minimum of about one hun-- dred amperes per square inch and a maximum of about one thousand amperes per square inch.

6. The method of claim 1, wherein said positive potential is applied repetitively and intermittently.

7. The method of claim 1, wherein said current density is about 1,000 amperes per square inch.

References Cited in the file of this patent UNITED STATES PATENTS Holbrook June 19, 1951 Turner July 17, 1956 OTHER REFERENCES Proc. of the I.R.E., December 1953, pp. 1706-1708, by Tiley et al. 

1. THE METHOD OF REMOVING A PORTION OF A BODY OF MATERIAL SELECTED FROM THE CLASS CONSISTING OF GERMANIUM, ARSENIC AND ANTIMONY, COMPRISING THE STEPS OF DIRECTING AGAINST SAID PORTION OF SAID BODY A JET OF AN ELECTROYTE CONTAINING HYDROGEN IONS WHILE MAINTIANING SAID JET POSITIVE WITH RESPECT TO SAID BODY TO PASS A CURRENT BETWEEEN SAID JET AND SAID BODY, SAID JET BEING MAINTAINED SUFFICIENTLY POSITIVE WITH RESPECT TO SAID BODY TO PRODUCE IN THE PORTION OF SAID JET IMPINGING SAID BODY PORTION AN AVERAGE ELECTRICAL CURRENT DENSITY OF AT LEAST 50 AMPERES PER SQUARE INCH AND TO CAUSE ETCHING OF SAID BODY PORTION BY SAID JET. 